The present invention relates to electricity power supply grids and, in particular, substations connecting the synchronised parts of a grid.
Modern Electricity Grids are inherently unstable. In most circumstances and for most of the time, the inherent instability can be contained and controlled by a variety of control actions and so, for most of us most of the time, the lights stay on.
Unfortunately, failures of equipment do occur. If the failure possibility has not been anticipated and planned is for in the configuration of the Grid, then it can trigger consequent changes in the behaviour of the Grid and flows of electricity that in turn can trigger a cascade of further failures. Various control strategies and actions are designed to minimise the possibility of such cascades, and to minimise their extent when they do occur. No known control strategy can provide guarantees against such failure cascades arising, or their potential extent. Total failure of a Grid and blackout of all supply remains a possibility, although effective implementation of good practice can make this prospect remote, and can shorten the period of a blackout. In Autumn of 2003 we have seen the consequences of this inherent instability in four significant blackouts. A huge blackout in the US, a short but high impact failure in London. A major failure in S. Scandinavia, and a blackout in Italy.
The inherent instability arises from fundamental features of electricity that have driven the Grid design:                Effective instantaneity of transmission. Electricity travels through wires at fractions of the speed of light. Although various equipment delays the propagation of effects, the effects will still propagate much faster than any status or control data needed if control is to react to actual events.        The synchronisation of the Grid. The instantaneity of transmission allows all components of the Grid to be locked in or synchronised to the single Grid frequency. The Alternating Current (or A/C) that is core to this is also the basis for many of the features that make A/C the fundamental basis of all electricity Grids.        
It is A/C that allowed the founders of the electricity industry to transmit power long distances, and to generate electricity at the large scale that made the early technologies more efficient. A/C can be transformed into very high voltages for efficient long distance transmission, and then transformed back to the lower voltages that are safe and useful for industrial or domestic uses. Subsequent evolution has now created Grids, all synchronised to the same common frequency, and a hierarchy of transmissions services (and thus voltages). Each level of the hierarchy links with lower and higher levels through substations.
Substations consist primarily of switches interconnecting the transformers which convert or transform the voltage. In general transformers are designed to provide one of two services:                “step up” transformers, so that the lower voltages at which electricity is generated is transformed into the high voltages of the long distance high capacity transmission network, or        distribution transformers, which convert from higher voltages to the lower voltages suitable for more local networks. Several distribution voltage levels will normally exist before reaching the 230 volts now standard for domestic and office use in Europe.        
Electricity Grids have generally been designed on the assumption that power will flow from the high voltage Grid, down through the hierarchy to local demand. Large generators will be providing the power, and are assumed to feed their power into the highest voltage levels, usually via step up transformers.
In Grids, electricity flows according to the physical properties of the network, and any or all electrical path between two points may carry electricity, albeit sometimes with its voltage transformed en route. With sufficient knowledge of the network; the sources and sinks of electricity; and the configuration of the switches the flows can be modelled and so predicted with reasonable accuracy.
There are means of influencing the resulting flows by active or passive control of voltage and reactive power. This control is exercised through generators (adjustment of gensets for voltage and power phasing); transformers (by choice of winding); by power converters (originally motor generator sets but more commonly now Static Converters or STATCONS), and even by judicious selection of transmission circuits.
Grid managers exercise much effort in predicting flows in event of particular component failures and thus involuntary reconfiguration of the network, and using these predictions to define and set configurations that are tolerant of component failures. Depending upon the planning and security objectives, these configurations are normally designed to avoid significant interruptions from any single possible failure. When failures do occur, replanning and manual switching to restore the levels of security become a high priority, so that once again the system is able to cope with single failures.
This replanning depends critically upon a proper understanding of what has actually happened. This understanding can often only be achieved by indirect means, depending upon a range of status signals fed back to the planners and operators. Such status signals are always slower than the effects they are measuring and can be inaccurate and unreliable so it can be hard or impossible to determine the root cause—the actual failure—from such indirect measurements. This makes automatic prediction and thus control too slow to be able to prevent cascades of failure.
Since the potential damage that can arise to the infrastructure from inappropriate electricity flows can be catastrophic for the equipment, automatic protections tend to be tuned to equipment protection, and this is most easily achieved by disconnection and shutdown—and thus blackout.
The scale of potential blackout extends to the whole Grid. So long as the Grids are interconnected by A/C, the flows in all parts are influenced by all other parts. If one part gets disconnected, this impacts the rest, and the configuration will often then be inappropriate for the new circumstances. So further overloads and failures are triggered.
In synchronised Grids as a whole, the frequency plays a central control role. It indicates the balance between generation and load. If there is too much generation, the frequency rises and if too little, it falls. A traditional role of the Grid manager is to ensure that there are controls in place, usually from the generators, so that the generation changes according to frequency. That is, if frequency rises, generators reduce output, and vice versa. An alternative and better way of managing the frequency is to have load devices that operate by a duty cycle, such as refrigerators, adjust their duty cycle according to the frequency. UK Patent Number GB2361118 describes such a system. This control is known as Response, and is a key pre-occupation of Grid managers to ensure that there is enough to respond to the normal random changes in demand as well as exceptional events, such as failures.
When failures cause part of a Grid to be lost or separated (this is known as islanding) from other part or parts of a Grid this will usually mean that the supply (generation) and demand (load) on the remaining part(s) is unbalanced. If there is not enough Response available, this imbalance will be reflected in changes in the frequency. If the frequency deviates too far from its central setting, then further protective relays will open, and further parts of the Grid will be lost.
If the response is not appropriately balanced across the Grid, then this can result in increased electricity flows at the points or circuits (ie substations) where different parts of the Grid are interconnected, and this is often a cause of a further failure in a cascade.
If two parts of a Grid become disconnected, then the frequency of the two parts will tend to differ, and the phasing of the A/C in the two parts will diverge. If there remain any interconnections between the two parts of the Grid (even if these are at quite different levels in the voltage hierarchy), this phase difference will cause unpredictable (and usually very harmful) behaviours where they meet. Later reconnection must also take place only when the two phases are coincident, and must have sufficient capacity to carry the flows that are needed to keep the two reconnected parts in on-going synchronisation. This is hard, and needs additional specialised equipment at various switching substations.
Until very recently, it has been the habit of Grid controllers to manage the system as a single Grid and a single frequency, and to shut down (and have automatic control systems that shut down) all generation and load in any separate islands that might form by separation from the “single” Grid. More recently, the possible reliability benefits of permitting separate islands to remain have been recognised, and there are efforts to form control systems that allow separate islands to survive. Certainly, higher densities of distributed generation make the single island control philosophy less appropriate.
Grids operating at different frequencies can be interconnected, and this is usually done by long distance DC circuits. These operate by converting the A/C from one Grid into DC; carrying the DC a short or long distance; and converting the DC back to A/C at the frequency of the receiving Grid. Recently great strides in the electronics to do this have been made, with semiconductors (transistors and diodes etc.) able to take larger and larger currents. This is a well established field, and so called “digital transformers” and power converters are available for many purposes and in many sizes.
Most DC interconnections are engineered to enable power to flow in either direction, commonly so that peak loads on one Grid can be met in part by power from another Grid whose peaks are a different times. This is most useful if the distance is substantial. Long distance DC transmission lines are used to connect the separate N American electricity Grids, and the cross channel link between the UK Grid and the European continental Grid is extensively used (albeit mostly to import electricity from France).
Power converters are also an increasingly important component of many generating and consuming devices. For example, many wind turbines will include power electronics to modulate their contribution to the Grid.
Substations exist wherever there is a connection between different levels in the Transmission and distribution hierarchy. At the top is the highest voltage level—the Transmission Network—operating in the UK at 400 kV. Other countries use different voltages. Most large generating stations inject electricity into the Grid at this level via step up transformers. This will carry electricity around the country to Grid substations. In most countries, the Transmission Grid has multiple voltage levels—in the UK the lower voltage Grid is 275 kV—and Grid Substations will transform the electricity for this network.
At each level, the Grid can, at least in theory, be partitioned into multiple separate networks interconnected via the higher level network. So separate 275 kV networks can run in different parts of the country. In practice, such networks usually have direct connections also.
The higher voltage transmission network supplies electricity to distribution networks at lower voltages via substations, usually with a further reduction in voltage. Grid Supply Points (GSPs) are also often points where the transfer is metered, and there may be a change in the ownership of the infrastructure now carrying the electricity.
Multiple distribution networks may be fed from a single GSP, and carry the electricity to the many points at which it is consumed, again through a succession of lower voltage networks, with substations where they interconnect.
Individual distribution networks may take their electricity from multiple GSPs, so removing their dependence upon any single supply point (and so ensuring they are unhurt by any single failure.) This invariably means that there is some sort of electrical path, outside the higher voltage transmission network, by which electricity can flow between different GSPs. These multiple electrical paths are one of the features that drive the desire to synchronise as much of the network as possible to the single Grid frequency.
The network control assumption has generally been that electricity flows from the high voltage network to the lower voltage one, reflecting the concept that big power stations achieved the greatest efficiencies. More recently the smaller generators, feeding electricity into the distribution network have become attractive. Electricity from this “embedded generation” is generally assumed to be consumed within the distribution network to which it is connected, doing no more than reducing the flows from the higher voltage network. However, as more distributed generation is installed, at whatever level in the network, this will, at times, generate more power than is consumed within the Distribution Network, so making export to the higher level Grid necessary.
This makes the Grid control problem even more complex and potentially unstable than the simple one way electricity flows. How should the embedded generation be controlled, and how should the electricity flows from distribution networks up to the transmission network be decided? What should happen when there is a fault in any of these networks, and how can these faults be minimised?
The present invention seeks to solve the problems identified above.